Their organs are very immature and the structure obscure. When they have quieted down somewhat in a preparation it may be seen that at the head end there is a six-lipped and very delicate prepuce, enclosing a short "fang" which may be suddenly exserted and retracted. Completely enclosing the larva is a delicate sheath, which is considerably longer than the worm itself. To enter into further details of anatomy is beyond the scope of this discussion and readers interested are referred to the work of Manson and of Fülleborn. One of the most surprising features of the habits of these larvæ is the periodicity which they exhibit in their occurrence in the peripheral blood. If a preparation be made during the day time there may be no evidence whatever of filarial infestation, whereas a preparation from the same patient taken late in the evening or during the night may be literally swarming with the parasites. Manson quotes Mackenzie as having brought out the further interesting fact that should a "filarial subject be made to sleep during the day and remain awake at night, the periodicity is reversed; that is to say, the parasites come into the blood during the day and disappear from it during the night." There have been numerous attempts to explain this peculiar phenomenon of periodicity but in spite of objections which have been raised, the most plausible remains that of Manson, who believes that it is an adaptation correlated with the life-habits of the liberating agent of the parasite, the mosquito. The next stages in the development of _Filaria nocturna_ occur in mosquitoes, a fact suggested almost simultaneously by Bancroft and Manson in 1877, and first demonstrated by the latter very soon thereafter. The experiments were first carried out with _Culex quinquefasciatus_ (= _fatigans_) as a host, but it is now known that a number of species of mosquitoes, both anopheline and culicine, may serve equally well. When the blood of an infested individual is sucked up and reaches the stomach of such a mosquito, the larvæ, by very active movements, escape from their sheaths and within a very few hours actively migrate to the body cavity of their new host and settle down primarily in the thoracic muscles. There in the course of sixteen to twenty days they undergo a metamorphosis of which the more conspicuous features are the formation of a mouth, an alimentary canal and a trilobed tail. At the same time there is an enormous increase in size, the larvæ which measured .3 mm. in the blood becoming 1.5 mm. in length. This developmental period may be somewhat shortened in some cases and on the other hand may be considerably extended. The controlling factor seems to be the one of temperature.